Fragile X syndrome is an X-linked dominant disorder representing the most common form of inherited mental retardation. Fragile X syndrome occurs with a frequency of approximately 1 in 4000 males and 1 in 8000 females. Affected males will exhibit moderate to severe mental retardation as well as a speech delay, hyperactivity, and behavioral and social difficulties. Fragile X syndrome results from the expansion of a CGG trinucleotide repeat in the FMR1 gene which encodes the FMRP protein. The CGG repeat resides in the 5'-untranslated region (UTR) of the FMR1 gene. Normal individuals have 6 to 53 CGG repeats and the premutation size is between 55 and 230 repeats. A full mutation in affected individuals is >200 repeats. Males with the premutation sized expansion develop a late-onset neurodegenerative condition termed fragile X-associated tremor/ataxia syndrome (FXTAS). In females with the premutation expansion there is a 25% chance of their developing ovarian failure. The molecular consequences of the amplification of the CGG repeat are the presentation of CpG dinucleotide sites that can be methylated on the cytidine. Indeed, the repeat expansion in the FMR1 gene leads to epigenetic transcriptional silencing due to hypermethylation of the promoter region of the gene.
Fragile X syndrome belongs to a family of disorders that are related to the relationship between fragile chromosomal sites and disease. Fragile chromosomal sites are only detectable in vitro when cells are exposed to chemical agents that disrupt the DNA replication process. To date 30 rare and 89 common fragile chromosome sites have been identified. Common sites are considered to be present in all individuals, whereas, rare sites are found only in a small percentage of persons. The majority of the rare fragile sites are folate-sensitive and are so defined because they present when cells are cultured in folate-deficient media or in the presence of inhibitors of folate metabolism. The fragile X syndrome locus (FRAXA) belongs to the class of folate-sensitive rare fragile sites. The majority of the common fragile sites are detectable by the addition of aphidicolin to cell cultures. Aphidicolin is an inhibitor of DNA polymerase.
The FMR1 gene is located on the X chromosome at Xq27.3 spanning 38 kb and composed of 17 exons that generate several alternatively spliced mRNAs. The different sized proteins encoded by these alternatively spliced mRNAs are localized to different subcellular locations. The FMRP protein is an RNA-binding protein found in ribonucleoprotein complexes associated with polyribosomes (polysomes). Early during human fetal deveopment the FMR1 gene is expressed at highest levels in cholinergic neurons of the nucleus basalis magnocellularis and in pyramidal neurons of the hippocampus which may contribute to the pathogenesis of fragile X syndrome. The FMR1 gene is also expressed in the mature testes at high levels in spermatogonia. The FMRP protein has two RNA-binding domains identified as K homology domains (KH domains). The domain was first identified in the heterogeneous nuclear ribonucleoprotein (hnRNP) K. FMRP has been shown to bind to approximately 4% of fetal human brain mRNAs. In addition, FMRP can bind to its own mRNA. Introducing a mutation into one of the two KH domains (KH2) has been shown to abolish the interaction of FMRP with polysomes and results in fragile X syndrome. The FMRP protein is found predominantly in the cytosol but it is shuttled between the nucleus and the cytoplasm. The ability of FMRP protein to associate with polysomes is suggestive of the protein being involved in the regulation of translation of target mRNAs. Indeed, several target mRNAs have been identified whose encoded proteins function in synaptic transmission, neuronal maturation and cytoskeletal structure. One important pathway involving the activity of FMRP is the metabotropic glutamate receptor-(mGluR) dependent long term depression (LTD) pathway. Glutamate receptors bind the amino acid glutamate which, in this capacity, functions as an excitatory neurotransmitter. The mGluRs function in both the central and peripheral nervous systems and are involved in memory, learning, anxiety and the perception of pain. Stimulation of the mGluR pathway leads to increased translation of FMRP, which subsequently suppresses the translation of other proteins at the synapse. The mechanism by which FMRP inhibits translation is related to the process of RNA degradation that involves the RNA-induced silencing complex (RISC). FMRP cooperates with microRNAs (miRNAs, which are small noncoding RNAs that form partial complimentary duplexes with regions of target mRNAs) resulting in activation of RISC. Two proteins that are components of the RISC are members of the Argonaute family (AGO1 and AGO2) and these proteins interact with FMRP. The interaction of the Argonaute proteins and FMRP is crucial for the function of FMRP.
The characteristic symptoms of fragile X syndrome in males is moderate to severe mental retardation. In males with the full mutation (i.e. >200 CGG repeats) the physical hallmarks of the disorder include a long narrow face, prominent ears and macroorchidism (large testicles). These physical signs are most apparent in postpubertal males. Additional physical findings can include hyperextensible finger joints, flat feet, double jointed thumbs and a high arched plate. Fragile X syndrome also manifests with distinct behavioral abnormalities that include hyperactivity, anxiety, extreme sensitivity to stimulation and sensations, hyperarousal and occasionally aggressive tendencies. The behavioral anomalies overlap with those of autism such as impaired verbal and nonverbal communication, difficulty with social interactions, poor eye contact, tactile defensiveness, hand flapping, and hand biting. Females with a full mutation are less affected with respect to all aspects of the mental and physical characteristics of fragile X syndrome. This is, of course, due to the process of X chromosome inactivation.